The aviation laws and regulations of many nations, including the United States, have created “highways” in the sky to which the flight paths of aircraft must conform. Except in emergency situations, aircraft may not deviate from approved flight paths without air traffic control (ATC) permission under Instrument Flight Rules (IFR). Such permission is conventionally requested and received from a local ATC facility with which an aircraft is in voice channel radio communication using onboard transmitter and receiver equipment. Traditionally, communications between any one aircraft and a corresponding local ATC facility occur over a line-of-sight communications path and over a dedicated frequency channel.
Aviation laws and regulations also prescribe requirements to avoid other aircraft. Airspace de-confliction describes the process of reducing the risk of a near mid-air collision. Although aircraft with conflicting routes may sometimes communicate and cooperate to adjust their routes and thus to avoid conflict, in some cases a conflicting aircraft cannot be contacted or is otherwise non-cooperative.
In the absence of an onboard pilot to perform conflict recognition and perform de-confliction communication, other operational or technical means are needed to detect and resolve potential airspace conflicts between aircraft. “Sense and avoid” systems in unmanned aircraft (UA) can assist in providing an acceptably safe means of compliance with regulations to detect and avoid other aircraft. These systems can be used for both self-separation and collision avoidance. Self-separation is the ability to remain “well clear” of other aircraft, typically through gentle, right-of-way compliant maneuvers. Collision avoidance is a function executed to prevent an imminent collision, and is typically more aggressive. The requirement to maintain “well clear” of other aircraft derives from regulatory language in U.S. Federal Aviation Regulations (FAR) governing general flight rules (14 C.F.R. §91.113).
Aircraft on IFR flight plans generally must ask for and receive permission to deviate from their planned flight paths, even to maneuver around traffic, unless a maneuver is needed to prevent a collision. A maneuver is considered to be needed to prevent a collision when the planned flight path would violate the well-clear distance, in view of information regarding the path of other aircraft.
ADS-B, which stands for Automatic Dependent Surveillance-Broadcast, is an Air Traffic Management and Control (ATM/ATC) surveillance system that was developed as a replacement for traditional radar-based systems for U.S. Next Generation Air Transportation System (NextGen)-like systems. An aircraft using ADS-B can determine its position via satellite navigation methodologies and periodically broadcast the determined position information, enabling the ADS-B-equipped aircraft to be tracked. For example, ADS-B avionics can broadcast messages over radio transmission links, approximately once per second, containing data signifying aircraft position, velocity, identification, and other ATC/ATM-related information. The data can be received by air traffic control ground stations as a replacement for secondary radar. It can also be received by other aircraft to provide situational awareness and allow self-separation and collision avoidance between airborne craft.
ADS-B is “automatic” in the sense that no pilot or controller action is required for the standard ADS-B information to be broadcast. It is “dependent surveillance” in the sense that the aircraft surveillance-type information is derived from on-board navigation equipment.